This invention relates to a new improved input planetary stage for a large gear box. The input planetary stage cited in this invention is for a wind turbine power generator having an output power capacity rating of 500 kW and greater.
Wind turbine power generators are considered one of the most cost effective and environmentally friendly methods of generating electricity. Individual wind turbines are currently being designed and built for electrical power generation in excess of 5 MW. A key component of most wind turbines are their gearboxes, which are subjected to varying high loading at low speeds, and have design lifetimes of 20 years. Anything that will give these gearboxes more durability and efficiency is highly coveted by wind turbine manufacturers and operators.
Modern large wind turbine generators (500 kW and greater) are massive devices commonly using large planetary gear systems as the input stage. These heavy gearboxes, which are mounted atop high towers, often located in remote locations such as on a mountain or offshore, experience severe fluctuations in wind conditions and temperature and are often exposed to a corrosive seawater environment and/or abrasive particulates. A gearbox failure can require removing the gearbox using mammoth equipment, and rebuilding it back at the manufacturer's facility followed by reinstallation at the remote location. The concurrent loss of electrical generation is also a costly event unto itself.
Manufacturers recognize that removing the peak asperities from the contact surfaces of gear teeth prior to full field operation increases the service life of the gearbox. There are two obvious advantages to removing peak asperities. Firstly, this will reduce the amount of metal-to-metal contact which produces lubricant debris and which is known to be destructive to gears and bearings. Secondly, it improves the material ratio (Rmr), which is a measure of the amount of gear tooth surface available for supporting the load. The industry assumed that any technique to remove the peak asperities was equivalent as long as no obvious metallurgical damage or no significant alteration to the lead and profile geometry occurred. Gear honing, for example, is often used in the aerospace industry to reduce peak asperity heights. Honing could have been a consideration for wind turbine gearboxes; except that it is cost prohibitive on such large gears as most honing equipment is limited to processing gears having a diameter of 12 inches or less. As such, today's wind turbine gears typically have ground teeth flanks and are recommended to be operated through a run-in procedure to remove the peak asperities from the contact surfaces.
It has been taught for a number of years that optimum performance benefits for bearings are achieved when the mating contacting surfaces are both isotropically superfinished to an arithmetic average roughness (Ra) of less than approximately 0.075 micron using chemically accelerated vibratory finishing. Similarly, gears in auto racing transmissions, which operate under high loads with high pitch line velocities, have benefited from this isotropic superfinishing process with teeth finishes of Ra from 0.3 micron to less than 0.025 micron. Such superfinished gears experience reduced contact fatigue, operating temperature, friction, noise and vibration.
Superfinishing enables the development of hydrodynamic lubrication (HL) or elastohydrodynamic lubrication (EHL). HL exists when there is complete separation of the mating gear teeth during operation achieved by a continuous lubricant film. EHL exists in highly loaded mated gear teeth under operation when the separating fluid film formation is influenced by elastic deformation of the contacting surfaces. Hence, with HL or EHL during their high speed and high load operation, superfinished auto racing transmissions experience almost no metal-to-metal contact of the mating teeth.
In sharp contrast to auto racing transmissions, the input planetary stage gears used in the wind turbine power generating industry operate under significantly different conditions. In wind turbine applications, the gears experience very high, varying loads at low pitch line velocities such that boundary lubrication rather than hydrodynamic (HL) or elastohydrodynamic lubrication (EHL) is predicted. Boundary lubrication exists when the mating gear teeth during operation are wetted with fluid but the lubricant film thickness is less than the combined mating surface roughness. Thus, the lubricant film can be penetrated by peak asperities, and metal-to-metal contact generates metal debris from the gear teeth. Traditionally manufactured ground wind turbine gear teeth (see “Standard for Design and Specification of Gear Boxes for Wind Turbines,” ANSI/AGMA/AWEA 6006-A03) after the run-in process described below, are hoped to achieve a surface finish of Ra=0.5-0.7 micron. However, those practiced in the art recognize that a traditionally manufactured hollow wheel will have a much higher surface finish. It is recommended by the AGMA standard that this gear's finish not exceed Ra>1.6 micron. Finishes of 0.5-0.7 micron are considered sufficient to avoid most metal to metal flank contact. It was also believed that this surface condition would result in significant lubricant retention needed with the slow moving gear teeth and thus the best possible lubrication condition would be achieved. However, a major source of wind turbine gear box failure is failure of the bearings. Even with run-in achieving the above finishes, metal to metal teeth contact continues on the planetary gear stage teeth and produces lubricant debris, which in turn contributes to the rapid bearing failures.
In contrast, chemically accelerated vibratory superfinishing to a condition of Ra<0.3 micron was thought to be too smooth for large wind turbine generators in that the teeth flanks would have insufficient lubricant retention for operation and tooth failure was predicted. Thus, it was questionable whether or not superfinishing using chemically accelerated vibratory finishing of the input planetary stage would add any performance value to the gear box. Only lengthy and costly field testing could provide the answer.
In addition, it was thought by those skilled-in-the-art that the large, heavy gears that make up an input planetary stage of a large wind turbine generator could not be processed in vibratory finishing equipment used in the chemically accelerated vibratory finishing process. This vibratory finishing equipment is either in a bowl or tub form. The input planetary stage gears are typically 200 kg or more for generators of an output capacity of 500 kW and larger. This gear weight was thought to be beyond the normal range of operation for vibratory finishing equipment.
In particular, it was thought that a large hollow wheel gear (annulus gear) weighing from 400 kg to greater than 5000 kg could not be superfinished in a large vibratory bowl. A person skilled-in-the-art would have predicted that such a massive gear with its relatively small cross sectional area would have immediately sunk to the bottom of the bowl damaging the lining, the gear or both. In addition, the heavy gear would have been expected to fracture significant quantities of the ceramic media used in the vibratory finishing equipment because of the high pressure exerted upon the media. The shards produced by the crushing of the ceramic media would have sharp points and edges. Instead of smoothing the critical contact surfaces of the gear teeth to a superfinished condition, these media fragments would have been predicted to damage these surfaces resulting in roughened, gouged and even denting the surfaces, especially nearer the bottom of the bowl where the pressure is greatest. The damage would have been significantly augmented for softer through-hardened (32-40 HRC) hollow wheel gears. The anticipated high rate of media attrition from fracturing would also add an unacceptable processing cost as well as causing the problem of clogging and blocking the drains of the processing machine.
Additionally, in processing the hollow wheel, it would have been expected that there would have been a variance in the intensity of media pressure across the lead of the gear teeth. The pressure of the media on the gear teeth nearer the bottom of the bowl is greater than the pressure of the media near the top. As a result, more stock would be expected to be removed from the gear teeth nearer the bottom than nearer the top. Therefore, the vibratory processed hollow wheel gear could end up being out of dimensional tolerance. This could be partially mitigated by removing the gear half way through the process, turning it over, returning it to the bowl, and continuing the process. It should be mentioned though that turning such a large gear is time consuming and potentially dangerous. Also, part of the center width of the gear teeth would be processed for twice the finishing time, possibly causing a resultant change in the tooth geometry. Each of the above predicted shortcomings would have been predicted to make this superfinishing process for large hollow wheel gears commercially impractical and unpredictable.
Similar shortcomings would have been expected for the chemically accelerated vibratory finishing of the other gears that make up the input planetary stage of a wind turbine gear box. These gears, known as planets and sun pinions, are similarly massive, typically weighing in excess of 200 kg each. As such, those skilled-in-the-art would have predicted they could not be processed in vibratory finishing equipment, whether bowls or tubs. Therefore, the wind turbine industry could not realize the benefits of this superfinishing process for the input planetary stage of the gear box.
It should be noted that it is desirous to be able to use through-hardened hollow wheel gears instead of gas nitrided or gas carburized hollow wheels in the large input planetary gear stage. Through-hardened hollow wheels are less costly to manufacture.
Gas nitriding is expensive, time consuming, and produces a very hard, brittle “white layer” on the teeth surfaces. Those practiced in the art recognize this white layer must be removed prior to use of the gear. However, removal of the white layer by grinding is at great expense and risk to ruining the hollow wheel. Alternative removal of the white layer by chemical dissolution is a very hazardous and environmentally unfriendly process.
In gas carburizing, due to the significant distortion from the heat treatment process, final grinding of the teeth is required, which is also an expensive process. Furthermore, after final grinding, the gas carburized hollow wheel requires temper burn inspection, another hazardous and environmentally unfriendly process.
Additionally, through-hardened hollow wheels are not just less expensive to manufacture, they can also be more geometrically accurate when compared to nitrided or carburized hollow wheels. This is very beneficial in that the remaining gears of the planetary gear set are routinely manufactured to high geometrical accuracy. Thus, if a more accurate, less expensive through-hardened hollow wheel can be operated with high accuracy planet and sun gears, the resulting planetary gear set could be highly efficient and of sufficient durability. If the through-hardened hollow wheel could be superfinished using chemically accelerated vibratory finishing, its teeth would be of sufficient surface capacity and capable of operating in HL or EHL regimes, thereby reducing debris generation. Thus, if superfinished through-hardened hollow wheels combined with superfinished planets and sun gears can operate satisfactorily at wind turbine designed loads and speeds, the result would be a superior input planetary gear stage. Alternatively, if the planets and sun pinion gears could be superfinished and mated to a non-superfinished hollow wheel, irrespective of its metallurgical heat treatment, the result would be an improved input planetary gear stage for a wind turbine generator of output capacity of 500 kW and greater. Therefore, superfinishing some, or preferably all, of the gears in the input planetary stage will result in a reduction or elimination of lubricant debris generated from the gear teeth, thereby reducing or eliminating a source of damage to the bearings.
In point of fact, gearbox manufacturers for large wind turbine power generators had only one viable choice for reducing peak asperities after grinding, and that was the run-in process. In the run-in process, the gears are smoothed in the assembled state by operating the gears box under various loads and speeds such that the contact area peak asperities shear away or plastic deform themselves. It should be mentioned that this was also the most economical route to take as the gearbox has to be tested and certified under load conditions anyway prior to its shipment and installation at its final destination. The run-in and testing phase can be conducted simultaneously on the same test rig. The AGMA/AWEA & The Danish Energy Agency, for example, have written guidelines for designing wind turbine gearboxes and stress the need for run-in. The impact of surface finish on gear tooth durability is briefly discussed, but the methodology of smoothing the surface is given no consideration. This view, that the method of removing the peak asperities is irrelevant, is generally shared by this industry as well as other gear manufacturers.
An ideal run-in process requires operation of the gearbox at different loads and speeds to simulate actual field conditions in order to smooth the asperity peaks across the whole load-carrying surface. However, duplicating actual service conditions on a test rig is not only virtually impossible, but is also impractical as well due to equipment, time and cost constraints. During the run-in process, the oil film thickness is often purposefully reduced to allow more asperity peak contact thereby resulting in a smoother surface. Once the run-in process is completed, the gearbox run-in lubricant and filtration system should be serviced. Typically the lubricant is drained, the gearbox flushed, and replaced with fresh lubricant, and the filter, which captures metal debris generated during the run-in process, is cleaned or replaced. Unfortunately, even during run-in, this metal debris can initiate serious damage to the bearings and gear contact surfaces before collection in a filter. And, most filters are capable of capturing only the largest debris particles and allow passage of the finer particles. These fine particles still are capable of causing surface damage, particularly to the gear box bearings.
Also, no matter how thoroughly or carefully the run-in procedure is conducted, this process leaves microscopic material distress (stress raisers) on the gear teeth contact regions due to the high stresses created to mechanically shear, fracture or elastically deform the peak asperities. These stress raisers act as initiation sites for future contact fatigue failures or micropitting.
Consequently, even after run-in, the input planetary gears often experience micropitting during the early period of field service. Micropitting by itself is another source of metal debris which can cause further damage to the bearings and gear contact surfaces since the metal debris is not immediately or completely trapped by the filtration system. It should be stressed that even microscopic metal debris particles, which can pass through a 10-micron filter, are still large enough to initiate damage. Micropitting is acknowledged to be an indicator of possible future gear failure and/or serious wear problems. Whenever severe wear occurs, the gear tooth profile is changed leading to increased vibration and noise which places an elevated stress on the gear box system.
Additionally, run-in procedures typically only smooth the drive side of the hollow wheel and sun gear while leaving the coast sides of these gear teeth as machined. During adverse operating conditions such as strong gusts of wind or turbine braking, coast side loading can be high enough to produce asperity contact and contribute to harmful metal debris. However, chemically accelerated vibratory superfinished gears are smoothed on both sides of the contact teeth surfaces.
Again, it needs to be emphasized that the industry has failed to give guidance on the actual optimum surface finish, or on the method of generating such optimum surfaces to improve gearbox durability. Instead it has relied mainly on run-in procedures to smooth the gear teeth contact areas to what was believed to be a satisfactory condition.